=head1 NAME

HTML::Mason::Devel - Mason Developer's Manual

=head1 DESCRIPTION

This manual is written for content developers who know HTML and at
least a little Perl. The goal is to write, run, and debug Mason
components.

If you are the webmaster (or otherwise responsible for the Mason
installation), you should also read L<the administrator's
manual|HTML::Mason::Admin>. There you will find information about site
configuration, performance tuning, component caching, and so on.

If you are a developer just interested in knowing more about Mason's
capabilities and implementation, then L<the administrator's
manual|HTML::Mason::Admin> is for you too.

We strongly suggest that you have a working Mason to play with as you
work through these examples. Other component examples can be found in
the C<samples/> directory.

While Mason can be used for tasks besides implementing a dynamic web
site, that is what I<most> people want to do with Mason, and is thus
the focus of this manual.

If you are planning to use Mason outside of the web, this manual will
still be useful, of course.  Also make sure to read the L<running
outside of mod_perl|HTML::Mason::Admin/running
outside of mod_perl> section of the administrator's manual.

=head1 HOW TO USE THIS MANUAL

If you are just learning Mason and want to get started quickly, we
recommend the following sections:

o L<What Are Components?|"What Are Components?">

o L<In-Line Perl Sections|"In-Line Perl Sections">

o L<Calling Components|"Calling Components">

o L<Top-Level Components|"Top-Level Components">

o L<Passing Parameters|"Passing Parameters">

o L<Initialization and Cleanup|"Initialization and Cleanup"> (mainly C<< <%init> >>)

o L<Web-Specific Features|"Web-Specific Features">

o L<Common Traps|"Common Traps">

=head1 WHAT ARE COMPONENTS?

The component - a mix of Perl and HTML - is Mason's basic building
block and computational unit. Under Mason, web pages are formed by
combining the output from multiple components.  An article page for a
news publication, for example, might call separate components for the
company masthead, ad banner, left table of contents, and article
body. Consider this layout sketch:

    +---------+------------------+
    |Masthead | Banner Ad        |
    +---------+------------------+
    |         |                  |
    |+-------+|Text of Article ..|
    ||       ||                  |
    ||Related||Text of Article ..|
    ||Stories||                  |
    ||       ||Text of Article ..|
    |+-------+|                  |
    |         +------------------+
    |         | Footer           |
    +---------+------------------+

The top level component decides the overall page layout, perhaps with
HTML tables. Individual cells are then filled by the output of
subordinate components, one for the Masthead, one for the Footer,
etc. In practice pages are built up from as few as one, to as many as
twenty or more components.

This component approach reaps many benefits in a web environment. The
first benefit is I<consistency>: by embedding standard design
elements in components, you ensure a consistent look and make it
possible to update the entire site with just a few edits. The second
benefit is I<concurrency>: in a multi-person environment, one person
can edit the masthead while another edits the table of contents.  A
last benefit is I<reuseability>: a component produced for one site
might be useful on another. You can develop a library of generally
useful components to employ on your sites and to share with others.

Most components emit chunks of HTML. "Top level" components, invoked
from a URL, represent an entire web page. Other, subordinate
components emit smaller bits of HTML destined for inclusion in top
level components.

Components receive form and query data from HTTP requests. When called
from another component, they can accept arbitrary parameter lists just
like a subroutine, and optionally return values.  This enables a type
of component that does not print any HTML, but simply serves as a
function, computing and returning a result.

Mason actually compiles components down to Perl subroutines, so you
can debug and profile component-based web pages with standard Perl
tools that understand the subroutine concept, e.g. you can use the
Perl debugger to step through components, and B<Devel::DProf> to
profile their performance.

=head1 IN-LINE PERL SECTIONS

Here is a simple component example:

    <%perl>
    my $noun = 'World';
    my @time = localtime;
    </%perl>
    Hello <% $noun %>,
    % if ( $time[2] < 12 ) {
    good morning.
    % } else {
    good afternoon.
    % }

After 12 pm, the output of this component is:

    Hello World, good afternoon.

This short example demonstrates the three primary "in-line" Perl
sections. In-line sections are generally embedded within HTML and
execute in the order they appear. Other sections (C<< <%init> >>,
C<< <%args> >>, etc.) are tied to component events like initialization,
cleanup, and argument definition.

The parsing rules for these Perl sections are as follows:

=over

=item 1.

Blocks of the form <% xxx %> are replaced with the result of
evaluating xxx as a single Perl expression.  These are often used for
variable replacement. such as 'Hello, <% $name %>!'.

=item 2.

Lines beginning with a '%' character are treated as Perl.

=item 3.

Multiline blocks of Perl code can be inserted with the C<< <%perl> >>
.. C<< </%perl> >> tag. The enclosed text is executed as Perl and the return
value, if any, is discarded.

The C<< <%perl> >> tag, like all block tags in Mason, is
case-insensitive. It may appear anywhere in the text, and may span any
number of lines.

=back

=head2 Examples and Recommended Usage

B<% lines>

Most useful for conditional and loop structures - if, while, foreach,
, etc. - as well as side-effect commands like assignments. To improve
readability, always put a space after the '%'. Examples:

o Conditional code

    % my $ua = $r->header_in('User-Agent');
    % if ($ua =~ /msie/i) {
    Welcome, Internet Explorer users
    ...
    % } elsif ($ua =~ /mozilla/i) {
    Welcome, Netscape users
    ...
    % }

o HTML list formed from array

    <ul>
    % foreach $item (@list) {
    <li><% $item %></li>
    % }
    </ul>

o HTML list formed from hash

    <ul>
    % while (my ($key,$value) = each(%ENV)) {
    <li>
    <b><% $key %></b>: <% $value %>
    </li>
    % }
    </ul>

o HTML table formed from list of hashes

    <table>
    % foreach my $h (@loh) {
    <tr>
    <td><% $h->{foo} %></td>
    <td bgcolor=#ee0000><% $h->{bar} %></td>
    <td><% $h->{baz} %></td>
    </tr>
    % }
    </table>

B<< <% xxx %> >>

Most useful for printing out variables, as well as more complex
expressions. To improve readability, always separate the tag
and expression with spaces. Examples:

  Dear <% $name %>: We will come to your house at <% $address %> in the
  fair city of <% $city %> to deliver your $<% $amount %> dollar prize!

  The answer is <% ($y+8) % 2 %>.

  You are <% $age < 18 ? 'not' : '' %> permitted to enter this site.

B<< <%perl> xxx </%perl> >>

Useful for Perl blocks of more than a few lines.

=head1 MASON OBJECTS

This section describes the various objects in the Mason universe.
If you're just starting out, all you need to worry about initially
are the request objects.

=head2 Request Objects

Two global per-request objects are available to all components: $r and
$m.

$r, the mod_perl request object, provides a Perl API to the current
Apache request.  It is fully described in Apache.pod. Here is a
sampling of methods useful to component developers:

    $r->uri             # the HTTP request URI
    $r->header_in(..)   # get the named HTTP header line
    $r->content_type    # set or retrieve content-type
    $r->header_out(..)  # set or retrieve an outgoing header

    $r->content         # don't use this one! (see Tips and Traps)

$m, the Mason request object, provides an analogous API for
Mason. Almost all Mason features not activated by syntactic tags are
accessed via $m methods.  You'll be introduced to these methods
throughout this document as they are needed.  For a description of all
methods see B<L<HTML::Mason::Request|HTML::Mason::Request>>.

Because these are always set inside components, you should not ever
define other variables with the same name, or else your code may fail
in strange and mysterious ways.

=head2 Component Objects

Mason provides an object API for components, allowing you to query a
component's various associated files, arguments, etc. For a description
of all methods see
B<L<HTML::Mason::Component|HTML::Mason::Component>>.  Typically you
get a handle on a component object from request methods like C<<
$m->current_comp >> and C<< $m->fetch_comp >>.

Note that for many basic applications all you'll want to do with
components is call them, for which no object method is needed. See
next section.

=head2 System Objects

Many system objects share the work of serving requests in Mason:
L<HTML::Mason::Lexer|HTML::Mason::Lexer>,
L<HTML::Mason::Compiler|HTML::Mason::Compiler>,
L<HTML::Mason::Interp|HTML::Mason::Interp>,
L<HTML::Mason::Resolver|HTML::Mason::Resolver>,
and L<HTML::Mason::ApacheHandler|HTML::Mason::ApacheHandler> are examples. The
administrator creates these objects and provides parameters that shape
Mason's behavior. As a pure component developer you shouldn't need to
worry about or access these objects, but occasionally we'll mention a
relevant parameter.

=head1 CALLING COMPONENTS

Mason pages often are built not from a single component, but from
multiple components that call each other in a hierarchical fashion.

=head2 Components that output HTML

To call one component from another, use the <& &> tag:

    <& comp_path, [name=>value, ...] &>

=over

=item comp_path:

The component path. With a leading '/', the path is relative to the
component root (L<comp_root|HTML::Mason::Params/comp_root>). Otherwise, it is relative to the
location of the calling component.

=item name => value pairs:

Parameters are passed as one or more C<< name => value >> pairs,
e.g. S<C<< player => 'M. Jordan' >>>.

=back

comp_path may be a literal string (quotes optional) or a Perl expression
that evaluates to a string. To eliminate the need for quotes in most
cases, Mason employs some magic parsing: If the first character is
one of C<[\w/_.]>, comp_path is assumed to be a literal
string running up to the first comma or &>. Otherwise, comp_path
is evaluated as an expression.

Here are some examples:

    # relative component paths
    <& topimage &>
    <& tools/searchbox &>

    # absolute component path
    <& /shared/masthead, color=>'salmon' &>

    # this component path MUST have quotes because it contains a comma
    <& "sugar,eggs", mix=>1 &>

    # variable component path
    <& $comp &>

    # variable component and arguments
    <& $comp, %args &>

    # you can use arbitrary expression for component path, but it cannot
    # begin with a letter or number; delimit with () to remedy this
    <& (int(rand(2)) ? 'thiscomp' : 'thatcomp'), id=>123 &>

Several request methods also exist for calling components.  C<< $m->comp >>
performs the equivalent action to <& &>:

    $m->comp('/shared/masthead', color=>'salmon');

C<< $m->scomp >> is like the sprintf version of C<< $m->comp >>: it returns
the component output, allowing the caller to examine and modify it
before printing:

    my $masthead = $m->scomp('/shared/masthead', color=>'salmon');
    $masthead =~ ...;
    $m->print($masthead);

=head2 Component Calls with Content

Components can be used to filter part of the page's content using an
extended component syntax.

    <&| /path/to/comp &> this is the content </&>
    <&| comp, arg1 => 'hi' &> filters can take arguments </&>
    <&| comp &> content can include <% "tags" %> of all kinds </&>
    <&| comp1 &> nesting is also <&| comp2 &> OK </&> </&>
    <&| SELF:method1 &> subcomponents can be filters </&>

The filtering component can be called in all the same ways a normal
component is called, with arguments and so forth.  The only difference
between a filtering component and a normal component is that a
filtering component is expected to fetch the content by calling
$m->content and do something with it.

The ending tag may optionally contain the name of the component, and Mason
will verify that it matches the name in the starting tag.  This may
be helpful when the tags are far apart or nested.  To avoid 
ambiguous situations, this is only allowed when the component name
is an unquoted literal (starting with C<[\w/_.]>).  For anything
more complicated, such as C<< <|& $var &> >> or C<< <&| 'name' &> >>,
the simple C<< </&> >> form must be used.

   <&| "outer" &>
     <&| /inner/comp, arg=>'this' &>
       <&| .mycomp &>
          Yada yada yada
       </& .mycomp >
     </& /inner/comp >
   </&>

Here is an example of a component used for localization.  Its content
is a series of strings in different languages, and it selects the
correct one based on a global C<$lang> variable, which could be setup
in a site-level autohandler.

   <&| /i18n/itext &>
      <en>Hello, <% $name %> This is a string in English</en>
      <de>Schoene Gruesse, <% $name %>, diese Worte sind auf Deutsch</de>
      <pig>ellohay <% substr($name,2).substr($name,1,1).'ay' %>,
      isthay isay igpay atinlay</pig>
   </&>

Here is the F</i18n/itext> component:

   <% $text %>

   <%init>
   # this assumes $lang is a global variable which has been set up earlier.
   local $_ = $m->content;
   my ($text) = m{<$lang>(.*?)</$lang>};
   </%init>

You can explicitly check whether a component has passed content by
checking the boolean C<< $m->has_content >>.  This allows you to write
a component that will do different things depending on whether it was
passed content. However, before overloading a component in this way,
consider whether splitting the behavior into two distinct components
would work as well.

If a normal component which does not call C<< $m->content >> is called
with content, the content will not be output.

If you wrap a filtering component call around the entire component,
the result will be functionally similar to a C<< <%filter> >> section.
See also L<Filtering|"Filtering">.

=head2 Advanced Components Calls with Content

Internally C<< $m->content >> is implemented with a closure containing
the part of the component which is the content.  In English, that
means that any mason tags and perl code in the content are evaluated
when C<< $m->content >> is called, and C<< $m->content >> returns the
text which would have been output by mason.  Because the contents are
evaluated at the time that C<< $m->content >> is called, one can write
components which act as control structures or which output their
contents multiple times with different values for the variables (can
you say taglibs?).

The tricky part of using filter components as control structures is
setting up variables which can be accessed from both the filter
component and the content, which is in the component which calls the
filter component.  The content has access to all variables in the
surrounding component, but the filtering component does not.  There
are two ways to do this: use global variables, or pass a reference to
a lexical variable to the filter component.

Here is a simple example using the second method:

    % my $var;
    <ol>
    <&| list_items , list => \@items, var => \$var &>
    <li> <% $var %></li>
    </&>
    </ol>

list_items component:

    <%args>
    @list
    $var
    </%args>
    % foreach (@list) {
    % $$var = $_;  # $var is a reference
    <% $m->content %>
    % }

Using global variables can be somewhat simpler.  Below is the same
example, with C<$var> defined as a global variable.  The site
administrator must make sure that C<$var> is included in Mason's
allow_globals parameter.  Local-izing C<$var> within the filter
component will allow the list_items component to be nested.

    <ol>
    <&| list_items, list => \@items &>
    <li> <% $var %></li>
    </&>
    </ol>

list_items component:

    <%args>
    @list
    </%args>
    % foreach (@list) {
    % local $var = $_;
    <% $m->content %>
    % }

Besides remembering to include C<$var> in allow_globals, the
developers should take care not to use that variable is other places
where it might conflict with usage by the filter component.
Local-izing $var will also provide some protection against using it in
other places.

An even simpler method is to use the C<$_> variable.  It is already
global, and is automatically local-ized by the foreach statement:

    <ol>
    <&| list_items, list => \@items &>
    <li> <% $_ %> </li>
    </&>
    </ol>

list_items component:

    <%args>
    @list
    </%args>
    % foreach (@list) {
    <% $m->content %>
    % }

=head2 Components that Return Values

So far you have seen components used solely to output HTML.  However,
components may also be used to return values.

While we will demonstrate how this is done, we strongly encourage you
to put code like this in modules instead.  There are several reasons
why this is a good idea:

=over 4

=item *

You can re-use this code outside of Mason.

=item *

It is easy to preload module code when running under mod_perl, which
can lower memory usage.

=item *

Using Mason components as subroutines is slower than just using
modules to do the same thing.

=item *

It's easier to regression test module code.

=back

With that being said, there are times when you may want to write a
component which returns a value.

As an example, you might have a component C<is_netscape> that analyzes
the user agent to determine whether it is a Netscape browser:

    <%init>
    my $ua = $r->header_in('User-Agent');
    return ($ua =~ /Mozilla/i && $ua !~ /MSIE/i) ? 1 : 0;
    </%init>

Because components are implemented underneath with Perl subroutines,
they can return values and even understand scalar/list
context. e.g. The result of wantarray() inside a component will reflect
whether the component was called in scalar or list context.

The <& &> notation only calls a component for its side-effect, and
discards its return value, if any.  To get at the return value of a
component, use the C<< $m->comp >> command:

    % if ($m->comp('is_netscape')) {
    Welcome, Netscape user!
    % }

Mason adds a C<return undef> to the bottom of each component to
provide an empty default return value. To return your own value from a
component, you I<must> use an explicit C<return> statement. You cannot
rely on the usual Perl trick of letting return values "fall through".

While it is possible for a component to generate output B<and> return
values, there is very little reason for a component to do both. For
example, it would not be very friendly for C<is_netscape> to output
"hi Mom" while it was computing its value, thereby surprising the
C<if> statement! Conversely, any value returned by an output
generating component would typically be discarded by the <& &> tag
that invoked it.

=head2 Subrequests

You may sometimes want to have a component call go through all the
steps that the initial component call goes through, such as checking
for autohandlers and dhandlers.  To do this, you need to execute a
subrequest.

A subrequest is simply a Mason Request object and has all of the
methods normally associated with one.

To create a subrequest you simply use the C<< $m->make_subrequest >>
method.  This method can take any parameters belonging to
L<HTML::Mason::Request|HTML::Mason::Request>, such as L<autoflush|HTML::Mason::Params/autoflush> or
L<out_method|HTML::Mason::Params/out_method>.  Once you have a new request object you simply call its
C<exec> method to execute it, which takes exactly the same parameters
as the C<comp> method.

Since subrequests inherit their parent request's parameters, output
from a component called via a subrequest goes to the same destination
as output from components called during the parent request.  Of course,
you can change this.

Here are some examples:

  <%perl>
   my $req = $m->make_subrequest( comp => '/some/comp', args => [ id => 172 ] );
   $req->exec;
  </%perl>

If you want to capture the subrequest's output in a scalar, you can
simply pass an L<out_method|HTML::Mason::Params/out_method> parameter to C<< $m->make_subrequest >>:

  <%perl>
   my $buffer;
   my $req =
       $m->make_subrequest
           ( comp => '/some/comp', args => [ id => 172 ], out_method => \$buffer );
   $req->exec;
  </%perl>

Now C<$buffer> contains all the output from that call to F</some/comp>.

For convenience, Mason also provides an C<< $m->subexec >> method.
This method takes the same arguments as C<< $m->comp >> and internally
calls C<< $m->make_subrequest >> and then C<exec> on the created
request, all in one fell swoop.  This is useful in cases where you
have no need to override any of the parent request object's
attributes.

By default, output from a subrequest appears inline in the calling
component, at the point where it is executed.  If you wish to do
something else, you will need to explicitly override the subrequest's
L<out_method|HTML::Mason::Params/out_method> parameter.

Mason Request objects are only designed to handle a single call to
C<exec>.  If you wish to make multiple subrequests, you must create
a new subrequest object for each one.

=head1 TOP-LEVEL COMPONENTS

The first component invoked for a page (the "top-level component")
resides within the DocumentRoot and is chosen based on the URL. For
example:

    http://www.foo.com/mktg/prods.html?id=372

Mason converts this URL to a filename,
e.g. F</usr/local/www/htdocs/mktg/prods.html>.  Mason loads and
executes that file as a component. In effect, Mason calls

    $m->comp('/mktg/prods.html', id=>372)

This component might in turn call other components and execute some Perl
code, or it might contain nothing more than static HTML.

=head2 dhandlers

What happens when a user requests a component that doesn't exist? In
this case Mason scans backward through the URI, checking each
directory for a component named I<dhandler> ("default handler").  If
found, the dhandler is invoked and is expected to use
C<< $m->dhandler_arg >> as the parameter to some
access function, perhaps a database lookup or location in another
filesystem. In a sense, dhandlers are similar in spirit to Perl's
AUTOLOAD feature; they are the "component of last resort" when a URL
points to a non-existent component.

Consider the following URL, in which F<newsfeeds/> exists but not the
subdirectory F<LocalNews> nor the component F<Story1>:

    http://myserver/newsfeeds/LocalNews/Story1

In this case Mason constructs the following search path:

    /newsfeeds/LocalNews/Story1         => no such thing
    /newsfeeds/LocalNews/dhandler       => no such thing
    /newsfeeds/dhandler                 => found! (search ends)
    /dhandler

The found dhandler would read "LocalNews/Story1" from
C<< $m->dhandler_arg >> and use it as a retrieval key into a
database of stories.

Here's how a simple /newsfeeds/dhandler might look:

    <& header &>
    <b><% $headline %></b><p>
    <% $body %>
    <& footer &>

    <%init>
    my $arg = $m->dhandler_arg;                # get rest of path
    my ($section, $story) = split("/", $arg);  # split out pieces
    my $sth = $DBH->prepare
        (qq{SELECT headline,body FROM news
            WHERE section = ? AND story = ?);
    $sth->execute($section, $story);
    my ($headline, $body) = $sth->fetchrow_array;
    return 404 if !$headline;                  # return "not found" if no such story
    </%init>

By default dhandlers do not get a chance to handle requests to a
directory itself (e.g. F</newsfeeds>). These are automatically
deferred to Apache, which generates an index page or a FORBIDDEN
error.  Often this is desirable, but if necessary the administrator
can let in directory requests as well; see the L<allowing directory
requests|HTML::Mason::Admin/allowing directory
requests> section of the administrator's manual.

A component or dhandler that does not want to handle a particular
request may defer control to the next dhandler by calling C<< $m->decline >>.

When using dhandlers under mod_perl, you may find that sometimes
Apache will not set a content type for a response.  This usually
happens when a dhandler handles a request for a non-existent file or
directory.  You can add a C<< <Location> >> or C<< <LocationMatch> >>
block containing a C<SetType> directive to your Apache config file, or
you can just set the content type dynamically by calling C<< $r->content_type >>.

The administrator can customize the file name used for dhandlers with
the L<dhandler_name|HTML::Mason::Params/dhandler_name> parameter.

=head2 autohandlers

Autohandlers allow you to grab control and perform some action just
before Mason calls the top-level component.  This might mean adding a
standard header and footer, applying an output filter, or setting up
global variables.

Autohandlers are directory based.  When Mason determines the top-level
component, it checks that directory and all parent directories for a
component called F<autohandler>. If found, the autohandler is called
first.  After performing its actions, the autohandler typically calls
C<< $m->call_next >> to transfer control to the original intended
component.

C<< $m->call_next >> works just like C<< $m->comp >> except that the component path
and arguments are implicit. You can pass additional arguments to
C<< $m->call_next >>; these are merged with the original arguments, taking
precedence in case of conflict.  This allows you, for example, to
override arguments passed in the URL.

Here is an autohandler that adds a common header and footer to each
page underneath its directory:

    <html>
    <head><title>McHuffy Incorporated</title></head>
    <body style="background-color: pink">

    % $m->call_next;

    <hr />
    Copyright 1999 McHuffy Inc.
    </body>
    </html>

Same idea, using components for the header/footer:

    <& /shared/header &>
    % $m->call_next;
    <& /shared/footer &>

The next autohandler applies a filter to its pages, adding an absolute
hostname to relative image URLs:

    % $m->call_next;

    <%filter>
    s{(<img[^>]+src=\")/} {$1http://images.mysite.com/}ig;
    </%filter>

Most of the time autohandler can simply call C<< $m->call_next >>
without needing to know what the next component is. However, should
you need it, the component object is available from
C<< $m->fetch_next >>. This is useful for calling the component manually,
e.g. if you want to suppress some original arguments or if you want to
use C<< $m->scomp >> to store and process the output.

If more than one autohandler applies to a page, each autohandler gets a chance to run.
The top-most autohandler runs first; each C<< $m->call_next >> transfers control to the
next autohandler and finally to the originally called component. This allows you, for
example, to combine general site-wide templates and more specific section-based templates.

Autohandlers can be made even more powerful in conjunction with
Mason's object-oriented style features: methods, attributes, and
inheritance.  In the interest of space these are discussed in a
separate section, L<Object-Oriented Techniques|"Object-Oriented
Techniques">.

The administrator can customize the file name used for autohandlers
with the L<autohandler_name|HTML::Mason::Params/autohandler_name> parameter.

=head2 dhandlers vs. autohandlers

dhandlers and autohandlers both provide a way to exert control over a
large set of URLs. However, each specializes in a very different
application.  The key difference is that dhandlers are invoked only
when no appropriate component exists, while autohandlers are invoked
only in conjunction with a matching component.

As a rule of thumb: use an autohandler when you have a set of
components to handle your pages and you want to augment them
with a template/filter. Use a dhandler when you want to create a set
of "virtual URLs" that don't correspond to any actual components,
or to provide default behavior for a directory.

dhandlers and autohandlers can even be used in the same directory. For
example, you might have a mix of real URLs and virtual URLs to which
you would like to apply a common template/filter.

=head1 PASSING PARAMETERS

This section describes Mason's facilities for passing parameters to
components (either from HTTP requests or component calls) and for
accessing parameter values inside components.

=head2 In Component Calls

Any Perl data type can be passed in a component call:

    <& /sales/header, s => 'dog', l => [2, 3, 4], h => {a => 7, b => 8} &>

This command passes a scalar ($s), a list (@l), and a hash (%h). The
list and hash must be passed as references, but they will be automatically
dereferenced in the called component.

=head2 In HTTP requests

Consider a CGI-style URL with a query string:

    http://www.foo.com/mktg/prods.html?str=dog&lst=2&lst=3&lst=4

or an HTTP request with some POST content. Mason automatically parses
the GET/POST values and makes them available to the component as
parameters.

=head2 Accessing Parameters

Component parameters, whether they come from GET/POST or another
component, can be accessed in two ways.

1.  Declared named arguments: Components can define an
C<< <%args> >> section listing argument names, types, and
default values. For example:

    <%args>
    $a
    @b       # a comment
    %c

    # another comment
    $d => 5
    $e => $d*2
    @f => ('foo', 'baz')
    %g => (joe => 1, bob => 2)
    </%args>

Here, I<$a>, I<@b>, and I<%c> are required arguments; the component generates
an error if the caller leaves them unspecified. I<$d>, I<$e>, I<@f> and I<%g> are
optional arguments; they are assigned the specified default values if
unspecified.  All the arguments are available as lexically scoped ("my")
variables in the rest of the component.

Arguments are separated by one or more newlines. Comments may be used at
the end of a line or on their own line.

Default expressions are evaluated in top-to-bottom order, and one
expression may reference an earlier one (as $e references $d above).

Only valid Perl variable names may be used in C<< <%args> >>
sections.  Parameters with non-valid variable names cannot be
pre-declared and must be fetched manually out of the %ARGS hash (see
below).  One common example of undeclarable parameters are the
"button.x/button.y" parameters sent for a form submit.

2. %ARGS hash: This variable, always available, contains all of the
parameters passed to the component (whether or not they were
declared).  It is especially handy for dealing with large numbers of
parameters, dynamically named parameters, or parameters with non-valid
variable names. %ARGS can be used with or without an
C<< <%args> >> section, and its contents are unrelated to what you
have declared in C<< <%args> >>.

Here's how to pass all of a component's parameters to another component:

    <& template, %ARGS &>

=head2 Parameter Passing Examples

The following examples illustrate the different ways to pass and receive parameters.

1.  Passing a scalar I<id> with value 5.

  In a URL: /my/URL?id=5
  In a component call: <& /my/comp, id => 5 &>
  In the called component, if there is a declared argument named...
    $id, then $id will equal 5
    @id, then @id will equal (5)
    %id, then an error occurs
  In addition, $ARGS{id} will equal 5.

2.  Passing a list I<colors> with values red, blue, and green.

  In a URL: /my/URL?colors=red&colors=blue&colors=green
  In an component call: <& /my/comp, colors => ['red', 'blue', 'green'] &>
  In the called component, if there is a declared argument named...
    $colors, then $colors will equal ['red', 'blue', 'green']
    @colors, then @colors will equal ('red', 'blue', 'green')
    %colors, then an error occurs
  In addition, $ARGS{colors} will equal ['red', 'blue', 'green'].

3.  Passing a hash I<grades> with pairs Alice => 92 and Bob => 87.

  In a URL: /my/URL?grades=Alice&grades=92&grades=Bob&grades=87
  In an component call: <& /my/comp, grades => {Alice => 92, Bob => 87} &>
  In the called component, if there is a declared argument named...
    @grades, then @grades will equal ('Alice', 92, 'Bob', 87)
    %grades, then %grades will equal (Alice => 92, Bob => 87)
  In addition, $grade and $ARGS{grades} will equal
    ['Alice',92,'Bob',87] in the URL case, or {Alice => 92, Bob => 87}
    in the component call case.  (The discrepancy exists because, in a
    query string, there is no detectable difference between a list or
    hash.)

=head2 Using @_ instead

If you don't like named parameters, you can pass a traditional
list of ordered parameters:

    <& /mktg/prods.html', 'dog', [2, 3, 4], {a => 7, b => 8} &>

and access them as usual through Perl's @_ array:

    my ($scalar, $listref, $hashref) = @_;

In this case no C<< <%args> >> section is necessary.

We generally recommend named parameters for the benefits of
readability, syntax checking, and default value automation.  However
using C<@_> may be convenient for very small components, especially
subcomponents created with C<< <%def> >>.

Before Mason 1.21, @_ contained I<copies> of the caller's arguments.
In Mason 1.21 and beyond, this unnecessary copying was eliminated and
@_ now contains I<aliases> to the caller's arguments, just as with
regular Perl subroutines. For example, if a component updates $_[0],
the corresponding argument is updated (or an error occurs if it is not
updateable).

Most users won't notice this change because C<< <%args> >> variables
and the C<%ARGS> hash always contain copies of arguments.

See perlsub for more information on @_ aliasing.

=head1 INITIALIZATION AND CLEANUP

The following sections contain blocks of Perl to execute at specific
times.

=head2 <%init>

This section contains initialization code that executes as soon as the
component is called. For example: checking that a user is logged in;
selecting rows from a database into a list; parsing the contents of a
file into a data structure.

Technically an C<< <%init> >> block is equivalent to a C<< <%perl> >>
block at the beginning of the component. However, there is an
aesthetic advantage of placing this block at the end of the component
rather than the beginning.

We've found that the most readable components (especially for
non-programmers) contain HTML in one continuous block at the top, with
simple substitutions for dynamic elements but no distracting blocks of
Perl code.  At the bottom an C<< <%init> >> block sets up the substitution
variables.  This organization allows non-programmers to work with the
HTML without getting distracted or discouraged by Perl code. For example:

    <html>
    <head><title><% $headline %></title></head>
    <body>
    <h2><% $headline %></h2>
    <p>By <% $author %>, <% $date %></p>

    <% $body %>

    </body>
    </html>

    <%init>
    # Fetch article from database
    my $dbh = DBI::connect ...;
    my $sth = $dbh->prepare("select * from articles where id = ?");
    $sth->execute($article_id);
    my ($headline, $date, $author, $body) = $sth->fetchrow_array;
    # Massage the fields
    $headline = uc($headline);
    my ($year, $month, $day) = split('-', $date);
    $date = "$month/$day";
    </%init>

    <%args>
    $article_id
    </%args>

=head2 <%cleanup>

This section contains cleanup code that executes just before the
component exits. For example: closing a database connection or closing
a file handle.

A C<< <%cleanup> >> block is equivalent to a C<< <%perl> >> block at
the end of the component. This means it will NOT execute if the
component explicitly returns, or if an abort or error occurs in that
component or one of its children. Because of this limitation, and
because Perl is usually so good about cleaning up at the end of a
lexical scope (e.g. component), C<< <%cleanup> >> sections are rarely
needed.

If you need code that is guaranteed to run when the component or
request exits, consider using a mod_perl cleanup handler, or creating
a custom class with a DESTROY method.

=head2 <%once>

This code executes once when the component is loaded. Variables
declared in this section can be seen in all of a component's code and
persist for the lifetime of the component.

This section is useful for declaring persistent component-scoped
lexical variables (especially objects that are expensive to create),
declaring subroutines (both named and anonymous), and initializing
state.

This code does not run inside a request context. You cannot call
components or access C<$m> or C<$r> from this section. Also, do not
attempt to C<return()> from a C<< <%once> >> section; the current
compiler cannot properly handle it.

Normally this code will execute individually from every HTTP child
that uses the component. However, if the component is preloaded, this
code will only execute once in the parent.  Unless you have total control
over what components will be preloaded, it is safest to avoid
initializing variables that can't survive a fork(), e.g. DBI handles.
Use code like this to initialize such variables in the C<< <%init> >>
section:

    <%once>
    my $dbh;      # declare but don't assign
    ...
    </%once>

    <%init>
    $dbh ||= DBI::connect ...
    ...
    </%init>

In addition, using C<$m> or C<$r> in this section will not work in a
preloaded component, because neither of those variable exist when a
component is preloaded.

=head2 <%shared>

As with C<< <%once> >>, lexical (C<my>) variables declared in this
section can be seen in all the rest of a component's code: the main
body, subcomponents, and methods.  However, unlike C<< <%once> >>, the
code runs once per request (whenever the component is used) and its
variables last only until the end of the request.

A C<< <%shared> >> section is useful for initializing variables needed in, say, the
main body and one more subcomponents or methods. See L<Object-Oriented
Techniques|"Object-Oriented Techniques"> for an example of usage.

It's important to realize that you do not have access to the C<%ARGS>
hash or variables created via an C<< <%args> >> block inside a shared
section.  However, you can access arguments via
L<$m-E<gt>request_args|HTML::Mason::Request/item_request_args>.

Additionally, you cannot call a components' own methods or
subcomponents from inside a C<< <%shared> >>, though you can call
other components.

Avoid using C<< <%shared> >> for side-effect code that needs to run at a
predictable time during page generation. You may assume only that
C<< <%shared> >> runs just before the first code that needs it and runs at
most once per request.

In the current implementation, the scope sharing is done with
closures, so variables will only be shared if they are visible at
compile-time in the other parts of the component.  In addition, you
can't rely on the specific destruction time of the shared variables,
because they may not be destroyed until the first time the C<<
<%shared> >> section executes in a future request.  C<< <%init> >>
offers a more predictable execution and destruction time.

Currently any component with a C<< <%shared> >> section incurs an
extra performance penalty, because Mason must recreate its
anonymous subroutines the first time each new request uses the
component.  The exact penalty varies between systems and for most
applications will be unnoticeable. However, one should avoid using
C<< <%shared> >> when patently unnecessary, e.g. when an C<< <%init> >>
would work just as well.

Do not attempt to C<return()> from a C<< <%shared> >> section; the
current compiler cannot properly handle it.

=head1 EMBEDDED COMPONENTS

=head2 <%def I<name>>

Each instance of this section creates a I<subcomponent> embedded
inside the current component. Inside you may place anything that a
regular component contains, with the exception of C<< <%def> >>, C<< <%method> >>,
C<< <%once> >>, and C<< <%shared> >> tags.

The I<name> consists of characters in the set C<[\w._-]>. To
call a subcomponent simply use its name in <& &> or C<< $m->comp >>. A
subcomponent can only be seen from the surrounding component.

If you define a subcomponent with the same name as a file-based
component in the current directory, the subcomponent takes
precedence. You would need to use an absolute path to call the
file-based component. To avoid this situation and for general clarity,
we recommend that you pick a unique way to name all of your
subcomponents that is unlikely to interfere with file-based
components. A commonly accepted practice is to start subcomponent
names with ".".

While inside a subcomponent, you may use absolute or relative paths to
call file-based components and also call any of your "sibling"
subcomponents.

The lexical scope of a subcomponent is separate from the main
component.  However a subcomponent can declare its own C<< <%args> >> section
and have relevant values passed in.  You can also use a C<< <%shared> >>
section to declare variables visible from both scopes.

In the following example, we create a ".link" subcomponent to produce a
standardized hyperlink:

    <%def .link>
    <a href="http://www.<% $site %>.com"><% $label %></a>

    <%args>
    $site
    $label=>ucfirst($site)
    </%args>
    </%def>

    Visit these sites:
    <ul>
     <li><& .link, site=>'yahoo' &></li>
     <li><& .link, site=>'cmp', label=>'CMP Media' &></li>
     <li><& .link, site=>'excite' &></li>
    </ul>

=head2 <%method I<name>>

Each instance of this section creates a I<method> embedded inside
the current component. Methods resemble subcomponents in terms of
naming, contents, and scope. However, while subcomponents can only be
seen from the parent component, methods are meant to be called from
other components.

There are two ways to call a method. First, via a path of the form
"comp:method":

    <& /foo/bar:method1 &>

    $m->comp('/foo/bar:method1');

Second, via the call_method component method:

    my $comp = $m->fetch_comp('/foo/bar');
    ...
    $comp->call_method('method1');

Methods are commonly used in conjunction with autohandlers to make
templates more flexible. See L<Object-Oriented
Techniques|"Object-Oriented Techniques"> for more information.

You cannot create a subcomponent and method with the same name. 
This is mostly to prevent obfuscation and accidental errors.

=head1 FLAGS AND ATTRIBUTES

The C<< <%flags> >> and C<< <%attr> >> sections consist of key/value
pairs, one per line, joined by '=>'.  In each pair, the key must be
any valid Perl "bareword identifier" (made of letters, numbers, and
the underscore character), and the value may be any scalar value,
including references.  An optional comment may follow each line.

=head2 <%flags>

Use this section to set official Mason flags that affect the current
component's behavior.

Currently there is only one flag, C<inherit>, which specifies the
component's I<parent> in the form of a relative or absolute component
path. A component inherits methods and attributes from its parent; see
L<Object-Oriented Techniques|"Object-Oriented Techniques"> for
examples.

    <%flags>
    inherit=>'/site_handler'
    </%flags>

=head2 <%attr>

Use this section to assign static key/value attributes that can be
queried from other components.

    <%attr>
    color => 'blue'
    fonts => [qw(arial geneva helvetica)]
    </%attr>

To query an attribute of a component, use the C<attr> method:

    my $color = $comp->attr('color')

where $comp is a component object.

Mason evaluates attribute values once when loading the component.
This makes them faster but less flexible than methods.

=head1 FILTERING

This section describes several ways to apply filtering functions over
the results of the current component.  By separating out and hiding a
filter that, say, changes HTML in a complex way, we allow
non-programmers to work in a cleaner HTML environment.

=head2 <%filter> section

The C<< <%filter> >> section allows you to arbitrarily filter the output of
the current component. Upon entry to this code, C<$_> contains the
component output, and you are expected to modify it in place. The code
has access to component arguments and can invoke subroutines, call
other components, etc.

This simple filter converts the component output to UPPERCASE:

    <%filter>
    tr/a-z/A-Z/
    </%filter>

The following navigation bar uses a filter to "unlink" and highlight
the item corresponding to the current page:

    <a href="/">Home</a> | <a href="/products/">Products</a> |
    <a href="/bg.html">Background</a> | <a href="/finance/">Financials</a> |
    <a href="/support/">Tech Support</a> | <a href="/contact.html">Contact Us</a>

    <%filter>
    my $uri = $r->uri;
    s{<a href="$uri/?">(.*?)</a>} {<b>$1</b>}i;
    </%filter>

This allows a designer to code such a navigation bar intuitively
without C<if> statements surrounding each link!  Note that the regular
expression need not be very robust as long as you have control over what
will appear in the body.

A filter block does not have access to variables declared in a
component's C<< <%init> >> section, though variables declared in the
C<< <%args> >>, C<< <%once> >> or C<< <%shared> >> blocks are
usable in a filter.

It should be noted that a filter cannot rely on receiving all of a
component's output at once, and so may be called multiple times with
different chunks of output.  This can happen if autoflush is on, or if
a filter-containing component, or the components it calls, call the
C<< $m->flush_buffer() >> method.

You should never call Perl's C<return()> function inside a filter
section, or you will not see any output at all.

You can use L<Component Calls with Content|"Component Calls with
Content"> if you want to filter specific parts of a component rather
than the entire component.

=head1 COMMENT MARKERS

There are several ways to place comments in components, i.e. arbitrary
text that is ignored by the parser.

=head2 <%doc>

Text in this section is treated as a comment and ignored. Most useful
for a component's main documentation.  One can easily write a program
to sift through a set of components and pull out their C<< <%doc> >>
blocks to form a reference page.

=head2 <% # comment... %>

A C<< <% %> >> tag is considered a comment if all of its lines are
either whitespace, or begin with a '#' optionally preceded by
whitespace. For example,

    <% # This is a single-line comment %>

    <%
       # This is a
       # multi-line comment
    %>

=head2 %# comment

Because a line beginning with C<%> is treated as Perl, C<%#>
automatically works as a comment. However we prefer the C<< <% #
comment %> >> form over C<< %# >>, because it stands out a little more
as a comment and because it is more flexible with regards to preceding
whitespace.

=head2 % if (0) { }

Anything between these two lines

   % if (0) {
   ...
   % }

will be skipped by Mason, including component calls.  While we don't
recommend this for comments per se, it is a useful notation for
"commenting out" code that you don't want to run.

=head2 HTML/XML/... comments

HTML and other markup languages will have their own comment markers, for example
C<< <!-- --> >>. Note two important differences with these comments versus the
above comments:

=over

=item *

They will be sent to the client and appear in the source of the page.

=item *

They do not block component calls and other code from running,
so don't try to use them to comment out code!

   <!-- Oops, the code below will still run
      <& /shared/expensive.mhtml &>
   -->

=back

=head1 OTHER SYNTAX

=head2 <%text>

Text in this section is printed as-is with all Mason syntax ignored.
This is useful, for example, when documenting Mason itself from a
component:

    <%text>
    % This is an example of a Perl line.
    <% This is an example of an expression block. %>
    </%text>

This works for almost everything, but doesn't let you output
C<< </%text> >> itself! When all else fails, use C<< $m->print >>:

    % $m->print('The tags are <%text> and </%text>.');

=head2 Escaping expressions

Mason has facilities for I<escaping> the output from C<< <% %> >>
tags, on either a site-wide or a per-expression basis.

Any C<< <% %> >> expression may be terminated by a '|' and one or more
escape flags (plus arbitrary whitespace), separated by commas:

    <% $file_data |h %>

The current valid flags are:

=over

=item * h

Escape HTML ('<' => '&lt;', etc.) using C<HTML::Entities::encode()>.
Before Perl 5.8.0 this module assumes that text is in the ISO-8859-1
character set; see L<the next section|HTML::Mason::Devel/"User-defined
Escapes"> for how to override this escaping. After 5.8.0, the encoding
assumes that text is in Unicode.

=item * u

Escape a URL query string (':' => '%3A', etc.) - all but
[a-zA-Z0-9_.-]

=item * n

This is a special flag indicating that the default escape flags should
I<not> be used for this substitution.

=back

The administrator may specify a set of default escape flags via the
L<default_escape_flags|HTML::Mason::Params/default_escape_flags> parameter. For example, if the administrator
sets L<default_escape_flags|HTML::Mason::Params/default_escape_flags> to C<['h']>, then all <% %> expressions
will automatically be HTML-escaped.  In this case you would use the
C<n> flag to turn off HTML-escaping for a specific expression:

    <% $html_block |n %>

Multiple escapes can be specified as a comma-separated list:

    <% $uri | u, n %>

The old pre-defined escapes, 'h', 'u', and 'n', can be used I<without>
commas, so that this is legal:

    <% $uri | un %>

However, this only works for these three escapes, and no others.  If
you are using user-defined escapes as well, you I<must> use a comma:

    <% $uri | u, add_session %>

=head3 User-defined Escapes

Besides the default escapes mentioned above, it is possible for the
user to define their own escapes or to override the built-in 'h' and
'u' escapes.

This is done via the Interp object's L<escape_flags|HTML::Mason::Params/escape_flags> parameter or
L<set_escape()|HTML::Mason::Interp/item_set_escape> method.  Escape
names may be any number of characters as long as it matches the regex
C</^[\w-]+$/>.  The one exception is that you cannot override the 'n'
flag.

Each escape flag is associated with a subroutine reference.  The
subroutine should expect to receive a scalar reference, which should
be manipulated in place.  Any return value from this subroutine is
ignored.

Escapes can be defined at any time but using an escape that is not
defined will cause an error when executing that component.

A common use for this feature is to override the built-in HTML
escaping, which will not work with non-ISO-8559-1 encodings.  If
you are using such an encoding and want to switch the 'h' flag to do
escape just the minimal set of characters (C<E<lt>>, C<E<gt>>, C<&>,
C<">), put this in your Apache configuration:

   PerlSetVar  MasonEscapeFlags  "h => \&HTML::Mason::Escapes::basic_html_escape"

Or, in a top-level autohandler:

    $m->interp->set_escape( h => \&HTML::Mason::Escapes::basic_html_escape );

Or you could write your own escape function for a particular encoding:

    $ah->interp->set_escape( h => \&my_html_escape );

And of course this can be used for all sorts of other things, like a
naughty words filter for the easily offended:

    $interp->set_escape( 'no-naughty' => \&remove_naughty_words );

=head3 Manually applying escapes

You can manually apply one or more escapes to text using the L<Interp
object's C<apply_escapes()>
method|HTML::Mason::Interp/item_apply_escapes>. e.g.

    $m->interp->apply_escapes( 'some html content', 'h' );

=head2 Backslash at end of line

A backslash (\) at the end of a line suppresses the newline. In HTML
components, this is mostly useful for fixed width areas like C<< <pre> >>
tags, since browsers ignore white space for the most part. An example:

    <pre>
    foo
    % if (1) {
    bar
    % }
    baz
    </pre>

outputs

    foo
    bar
    baz

because of the newlines on lines 2 and 4. (Lines 3 and 5 do not
generate a newline because the entire line is taken by Perl.)
To suppress the newlines:

    <pre>
    foo\
    % if (1) {
    bar\
    % }
    baz
    </pre>

which prints

    foobarbaz

=head1 DATA CACHING

Mason's data caching interface allows components to cache the results
of computation for improved performance.  Anything may be cached, from
a block of HTML to a complex data structure.

Each component gets its own private, persistent data cache. Except
under special circumstances, one component does not access another
component's cache. Each cached value may be set to expire at a certain
time.

Data caching is implemented on top of one of two external caching APIs:
C<Cache::Cache>, which is stable but has not changed in years, or C<CHI>,
which has picked up where C<Cache::Cache> has left off and is actively
maintained. You control which one Mason uses with the L<data_cache_api|HTML::Mason::Params/data_cache_api>
parameter.  C<Cache::Cache> is the default for backward compatibility
reasons, but we recommend C<CHI> for anyone doing serious caching.  The APIs
are very similar for Mason users, so that most of the information below
applies to both; any differences are noted.

=head2 Basic Usage

The C<< $m->cache >> method returns a cache object representing the cache for
this component. Here's the typical usage of C<< $m->cache >>:

    my $result = $m->cache->get('key');
    if (!defined($result)) {
        ... compute $result ...
        $m->cache->set('key', $result);
    }

C<< $m->cache->get >> attempts to retrieve this component's cache
value. If the value is available it is placed in C<$result>. If the
value is not available, C<$result> is computed and stored in the
cache by C<< $m->cache->set >>.

=head2 Multiple Keys/Values

A cache can store multiple key/value pairs. A value can be
anything serializable by C<Storable>, from a simple scalar to an
arbitrary complex list or hash reference:

    $m->cache->set(name => $string);
    $m->cache->set(friends => \@list);
    $m->cache->set(map => \%hash);

You can fetch all the keys in a cache with

    my @idents = $m->cache->get_keys;

It should be noted that Mason reserves all keys beginning with
C<__mason> for its own use.

=head2 Expiration

You can pass an optional third argument to C<< $m->cache->set >>
indicating when the item should expire:

    $m->cache->set('name1', $string1, '5 min');  # Expire in 5 minutes
    $m->cache->set('name2', $string2, '3h');     # Expire in 3 hours

To change the expiration time for a piece of data, call C<set> again
with the new expiration. To expire an item immediately, use
C<< $m->cache->remove >>.

You can also specify an expiration condition when you fetch the item,
using the I<expire_if> option:

    my $result = $m->cache->get('key',
        expire_if=>sub { $_[0]->get_created_at < (stat($file))[9] });

I<expire_if> takes an anonymous subroutine, which is called with the
L<cache object|"Cache Object"> as its only parameter. If the
subroutine returns a true value, the item is expired. In the example
above, we expire the item whenever a certain file changes.

Finally, you can expire a cache item from an external script; see
L<Accessing a Cache Externally|"Accessing a Cache Externally"> below.

=head2 Avoiding Concurrent Recomputation

The code shown in "Basic Usage" above,

   my $result = $m->cache->get('key');
   if (!defined($result)) {
       ... compute $result ...
       $m->cache->set('key', $result);
   }

can suffer from a kind of race condition for caches that
are accessed frequently and take a long time to recompute.

Suppose that a particular cache value is accessed five times a
second and takes three seconds to recompute.  When the cache expires,
the first process comes in, sees that it is expired, and starts to
recompute the value.  The second process comes in and does the same thing.
This sequence continues until the first process finishes and stores
the new value.  On average, the value will be recomputed and written
to the cache 15 times!

One solution is the I<busy_lock> flag:

   my $result = $m->cache->get('key', busy_lock=>'30 sec');

In this case, when the value cannot be retrieved, C<get()> sets
the expiration time of the value 30 seconds in the future before
returning C<undef>.  This tells the first process to compute the new
value while causing subsequent processes to use the old value for 30
seconds. 

Should the 30 seconds expire before the first process is done, a second
process will start computing the new value while setting the expiration
time yet another 30 seconds in the future, and so on.

The disadvantage of this solution is that multiple writes to the cache
will be performed for each C<set()>.

Another solution, available only if you are using C<CHI>, is
C<expires_variance> which will create a variable time window during
which expiration may occur. See the C<CHI> documentation for details.

=head2 Caching All Output

Occasionally you will need to cache the complete output of a
component.  For this purpose, Mason offers the C<< $m->cache_self >>
method.  This method causes Mason to check to see if this component
has already been run and its output cached.  If this is the case, this
output is simply sent as output.  Otherwise, the component run
normally and its output and return value cached.

It is typically used right at the top of an C<< <%init> >> section:

    <%init>
    return if $m->cache_self(key => 'fookey', expires_in => '3 hours',
                             ... <other cache options> ...);
     ... <rest of init> ...
    </%init>

A full list of parameters and examples are available in the
L<cache_self|HTML::Mason::Request/item_cache_self> section of the Request
manual.

=head2 Cache Object

C<< $m->cache->get_object >> returns a C<Cache::Object> or
C<CHI::CacheObject> associated with a particular key. You can use this
to retrieve useful meta-data:

    my $co = $m->cache->get_object('name1');
    $co->get_created_at();    # when was object stored in cache
    $co->get_expires_at();    # when does object expire

=head2 Choosing a Cache Subclass - with Cache::Cache

The C<Cache::Cache> API is implemented by a variety of backend subclasses. For
example, C<FileCache> implements the interface with a set of
directories and files, C<MemoryCache> implements the interface in
process memory, and C<SharedMemoryCache> implements the interface in
shared memory.

By default C<< $m->cache >> uses C<FileCache>, but you can override
this with the I<cache_class> keyword. The value must be the name of a
C<Cache::Cache> subclass; the prefix "Cache::" need not be included.
For example:

    my $result = $m->cache(cache_class => 'MemoryCache')->get('key');
    $m->cache(cache_class => 'MemoryCache')->set(key => $result);

You can even specify different subclasses for different keys in the
same component. Just make sure the correct value is passed to all
calls to C<< $m->cache >>; Mason does not remember which subclass you
have used for a given component or key.

The administrator can set the default cache subclass used by all
components with the L<data_cache_defaults|HTML::Mason::Params/data_cache_defaults> parameter.

=head2 Choosing a Cache Subclass - with CHI

The C<CHI> API is implemented by a variety of drivers, for example
C<CHI::Driver::File>, C<CHI::Driver::FastMmap>, and
C<CHI::Driver::Memcached>.

C<CHI::Driver::File> is the default, but you can override this with
the I<driver> keyword. The value must be the name of a C<CHI::Driver>
subclass; the prefix "CHI::Driver::" need not be included.  For
example:

    my $cache = $m->cache(driver => 'Memcached', servers => [ ... ]);
    my $result = $cache->get('key');
    $cache->set(key => $result);

You can even specify different subclasses for different keys in the
same component. Just make sure the correct value is passed to all
calls to C<< $m->cache >>; Mason does not remember which subclass you
have used for a given component or key.

The administrator can set the default cache subclass used by all
components with the L<data_cache_defaults|HTML::Mason::Params/data_cache_defaults> parameter.

=head2 Accessing a Cache Externally

To access a component's cache from outside the component (e.g. in an
external Perl script), you'll need have the following information:

=over

=item *

the namespace associated with the component. For C<Cache::Cache>, the function
C<HTML::Mason::Utils::data_cache_namespace>,
given a component id (usually just the component path), returns the
namespace. For C<CHI>, the component id/path itself is the namespace.

=item *

the cache_root, for file-based caches only. Defaults to the
"cache" subdirectory under the Mason data directory.

=back

Given this information you can get a handle on the component's cache.
For example, the following code removes a cache item for component
F</foo/bar>, assuming the data directory is F</usr/local/www/mason>
and you are using the default file backend:

    use HTML::Mason::Utils qw(data_cache_namespace);

    # With Cache::Cache
    my $cache = new Cache::FileCache
        ( { namespace => data_cache_namespace("/foo/bar"),
            cache_root => "/usr/local/www/mason/cache" } );

    # With CHI
    my $cache = CHI->new
        ( driver => 'File',
          namespace => "/foo/bar",
          cache_root => "/usr/local/www/mason/cache" );

    # Remove one key
    $cache->remove('key1');

    # Remove all keys
    $cache->clear;

=head2 Mason 1.0x Cache API

For users upgrading from 1.0x and earlier, any existing $m-E<gt>cache
code will be incompatible with the new API. However, if you wish to
continue using the 1.0x cache API for a while, you (or your
administrator) can set L<data_cache_api|HTML::Mason::Params/data_cache_api> to '1.0'. All of the
$m-E<gt>cache options with the exception of C<tie_class> should be
supported.

The C<access_data_cache> function is no longer available; this will
need to be converted to use C<Cache::Cache> directly, as described in
the L<previous section|HTML::Mason::Devel/Accessing a Cache
Externally>.

=head1 WEB-SPECIFIC FEATURES

=head2 Sending HTTP Headers

Mason automatically sends HTTP headers via C<< $r->send_http_header >>
but it will not send headers if they've already been sent manually.

To determine the exact header behavior on your system, you need to
know whether your server's default is to have L<autoflush|HTML::Mason::Params/autoflush> on or off.
Your administrator should have this information.  If your administrator
doesn't know then it is probably off, the default.

With autoflush off the header situation is extremely simple: Mason
waits until the very end of the request to send headers. Any component
can modify or augment the headers.

With autoflush on the header situation is more complex.  Mason will
send headers just before sending the first output.  This means that if
you want to affect the headers with autoflush on, you must do so
before any component sends any output.  Generally this takes place in
an C<< <%init> >> section.

For example, the following top-level component calls another component
to see whether the user has a cookie; if not, it inserts a new cookie
into the header.

    <%init>
    my $cookie = $m->comp('/shared/get_user_cookie');
    if (!$cookie) {
        $cookie = new CGI::Cookie (...);
        $r->header_out('Set-cookie' => $cookie);
    }
    ...
    </%init>

With autoflush off this code will always work.  Turn autoflush on and
this code will only work as long as F</shared/get_user_cookie> doesn't
output anything (given its functional nature, it shouldn't).

The administrator can turn off automatic header sending via the
L<auto_send_headers|HTML::Mason::Params/auto_send_headers> parameter. You can also turn it off on
individual pages with

    $m->auto_send_headers(0);

=head2 Returning HTTP Status

The value returned from the top-most component becomes the status code
of the request. If no value is explicitly returned, it defaults to OK
(0).

Simply returning an error status (such as 404) from the top-most
component has two problems in practice. First, the decision to return
an error status often resides further down in the component
stack. Second, you may have generated some content by the time this
decision is made. (Both of these are more likely to be true when using
autohandlers.)

Thus the safer way to generate an error status is

   $m->clear_buffer;
   $m->abort($status);

C<< $m->abort >> bypasses the component stack and ensures that
C<$status> is returned from the top-most component. It works by
throwing an exception. If you wrapped this code (directly or
indirectly) in an eval, you must take care to rethrow the exception,
or the status will not make it out:

   eval { $m->comp('...') };
   if (my $err = $@) {
      if ($m->aborted) {
          die $err;
      } else {
          # deal with non-abort exceptions
      }
   }

=head3 Filters and $m->abort

A filter section will still be called after a component aborts with
C<< $m->abort >>.  You can always check C<< $m->aborted >> in your
C<< <%filter> >> block if you don't want to run the filter after an abort.

  <%filter>
  unless ( $m->aborted ) {
      $_ .= ' filter stuff';
  }
  </%filter>

=head2 External Redirects

Because it is so commonly needed, Mason 1.1x and on provides an
external redirect method:

    $m->redirect($url);    # Redirects with 302 status

This method uses the clear_buffer/abort technique mentioned above,
so the same warnings apply regarding evals.

Also, if you generate any output I<after> calling C<< $m->redirect >>,
then this output will be sent, and will break the redirect.  For
example:

  % eval { $m->comp('redirect', ...) };

  % die $@ if $@;

The blank line between the two Perl lines is new output generated
after the redirect.  Either remove it or call C<< $m->clear_buffer >>
immediately before calling C<die()>.

=head2 Internal Redirects

There are two ways to perform redirects that are invisible to the
client.

First, you can use a Mason subrequest (see L</Subrequests>). This only
works if you are redirecting to another Mason page.

Second, you can use Apache's internal_redirect method, which works
whether or not the new URL will be handled by Mason.  Use it this way:

    $r->internal_redirect($url);
    $m->auto_send_headers(0);
    $m->clear_buffer;
    $m->abort;

The last three lines prevent the original request from accidentally
generating extra headers or content.

=head1 USING THE PERL DEBUGGER

You can use the perl debugger in conjunction with a live
mod_perl/Mason server with the help of Apache::DB, available from
CPAN. Refer to the Apache::DB documentation for details.

The only tricky thing about debugging Mason pages is that components
are implemented by anonymous subroutines, which are not easily
breakpoint'able. To remedy this, Mason calls the dummy subroutine
C<debug_hook> at the beginning of each component. You can breakpoint
this subroutine like so:

    b HTML::Mason::Request::debug_hook

debug_hook is called with two parameters: the current Request object
and the full component path. Thus you can breakpoint specific
components using a conditional on $_[1]:

    b HTML::Mason::Request::debug_hook $_[1] =~ /component name/

You can avoid all that typing by adding the following to your
~/.perldb file:

    # Perl debugger aliases for Mason
    $DB::alias{mb} = 's/^mb\b/b HTML::Mason::Request::debug_hook/';

which reduces the previous examples to just: 

    mb
    mb $_[1] =~ /component name/

Mason normally inserts '#line' directives into compiled components so
that line numbers are reported relative to the source file. Depending
on your task, this can be a help or a hindrance when using the
debugger.  The administrator can turn off '#line' directives with
the L<use_source_line_numbers|HTML::Mason::Params/use_source_line_numbers> parameter.

=head1 LOGGING

Mason uses C<Log::Any> to log various events, such as the start and
end of each request and each component call. You can also log to
C<Log::Any> from a component with the
C<$m-E<gt>log> method. e.g.

    $m->log->error("Something bad happened!");
    $m->log->debugf("Arguments for '%s' were '%s'", $func, \%args)
        if $m->log->is_debug;

See C<Log::Any::Adapter> for how to direct these logs to
an output of your choice.

=head1 OBJECT-ORIENTED TECHNIQUES

Earlier you learned how to assign a common template to an entire
hierarchy of pages using I<autohandlers>. The basic template looks like:

    header HTML
    % $m->call_next;
    footer HTML

However, sometimes you'll want a more flexible template that adjusts
to the requested page.  You might want to allow each page or
subsection to specify a title, background color, or logo image while
leaving the rest of the template intact. You might want some pages or
subsections to use a different template, or to ignore templates
entirely.

These issues can be addressed with the object-oriented style
primitives introduced in Mason 0.85.

Note: we use the term object-oriented loosely. Mason borrows concepts
like inheritance, methods, and attributes from object methodology but
implements them in a shallow way to solve a particular set of
problems. Future redesigns may incorporate a deeper object
architecture if the current prototype proves successful.

=head2 Determining inheritance

Every component may have a single I<parent>. The default parent is a
component named C<autohandler> in the closest parent directory.  This
rule applies to autohandlers too: an autohandler may not have itself
as a parent but may have an autohandler further up the tree as its
parent.

You can use the C<inherit> flag to override a component's parent:

    <%flags>
    inherit => '/foo/bar'
    </%flags>

If you specify undef as the parent, then the component inherits from
no one.  This is how to suppress templates.

Currently there is no way to specify a parent dynamically at run-time,
or to specify multiple parents.

=head2 Content wrapping

At page execution time, Mason builds a chain of components from the
called component, its parent, its parent's parent, and so
on. Execution begins with the top-most component; calling
C<< $m->call_next >> passes control to the next component in the chain.  This
is the familiar autohandler "wrapping" behavior, generalized for any
number of arbitrarily named templates.

=head2 Accessing methods and attributes

A template can access methods and/or attributes of the requested
page. First, use C<< $m->request_comp >> to get a handle on the
appropriate component:

    my $self = $m->request_comp;

$self now refers to the component corresponding to the requested page
(the component at the end of the chain).

To access a method for the page, use C<call_method>:

    $self->call_method('header');

This looks for a method named 'header' in the page component.  If no
such method exists, the chain of parents is searched upwards, until
ultimately a "method not found" error occurs. Use 'method_exists' to
avoid this error for questionable method calls:

    if ($self->method_exists('header')) { ...

The component returned by the C<< $m->request_comp >> method never
changes during request execution.  In contrast, the component returned
by C<< $m->base_comp >> may change several times during request
execution.

When execution starts, the base component is the same as the requested
component.  Whenever a component call is executed, the base component
may become the component that was called.  The base component will
change for all component calls B<except> in the following cases:

=over

=item *

A component is called via its component object rather than its path,
for example:

  <& $m->fetch_comp('/some/comp'), foo => 1 &>

=item *

A subcomponent (defined with C<< <%def> >>) is called.

=item *

A method is called via the use of C<SELF:>, C<PARENT:>, or
C<REQUEST:>.  These are covered in more detail below.

=back

In all other cases, the base component is the called component or the
called component's owner component if that called component is a
method.

As hinted at above, Mason provides a shortcut syntax for method calls.

If a component call path starts with C<SELF:>, then Mason will start
looking for the method (the portion of the call after C<SELF:>), in
the base component.

    <& SELF:header &>
    $m->comp('SELF:header')

If the call path starts with C<PARENT:>, then Mason will start looking
in the current component's parent for the named method.

    <& PARENT:header &>
    $m->comp('PARENT:header')

In the context of a component path, PARENT is shorthand for
C<< $m->current_comp->parent >>.

If the call path begins with C<REQUEST:>, then Mason looks for the
method in the requested component.  REQUEST is shorthand for C<<
$m->request_comp >>.

The rules for attributes are similar. To access an attribute for the
page, use C<attr>:

    my $color = $self->attr('color')

This looks for an attribute named 'color' in the $self component. If
no such attribute exists, the chain of parents is searched upwards,
until ultimately an "attribute not found" error occurs. Use
C<attr_exists> or C<attr_if_exist> to avoid this error for
questionable attributes:

    if ($self->attr_exists('color')) { ...

    my $color = $self->attr_if_exists('color'); # if it doesn't exist $color is undef

=head2 Sharing data

A component's main body and its methods occupy separate lexical
scopes. Variables declared, say, in the C<< <%init> >> section of the main
component cannot be seen from methods.

To share variables, declare them either in the C<< <%once> >> or C<< <%shared> >>
section. Both sections have an all-inclusive scope. The C<< <%once> >>
section runs once when the component loads; its variables are
persistent for the lifetime of the component. The C<< <%shared> >> section
runs once per request (when needed), just before any code in the
component runs; its variables last only til the end of the request.

In the following example, various sections of code require information
about the logged-in user. We use a C<< <%shared> >> section to fetch these
in a single request.

    <%attr>
    title=>sub { "Account for $full_name" }
    </%attr>

    <%method lefttoc>
    <i><% $full_name %></i>
    (<a href="logout.html">Log out</a>)<br />
    ...
    </%method>

    Welcome, <% $fname %>. Here are your options:

    <%shared>
    my $dbh = DBI::connect ...;
    my $user = $r->connection->user;
    my $sth = $dbh->prepare("select lname,fname, from users where user_id = ?");
    $sth->execute($user);
    my ($lname, $fname) = $sth->fetchrow_array;
    my $full_name = "$first $last";
    </%shared>

C<< <%shared> >> presents a good alternative to C<< <%init> >> when data is needed
across multiple scopes. Outside these situations, C<< <%init> >> is preferred
for its slightly greater speed and predictable execution model.

=head2 Example

Let's say we have three components:

    /autohandler
    /products/autohandler
    /products/index.html

and that a request comes in for /products/index.html.

F</autohandler> contains a general template for the site, referring to a
number of standard methods and attributes for each page:

    <head>
    <title><& SELF:title &></title>
    </head>
    <body style="<% $self->attr('body_style') %>">
    <& SELF:header &>

    <div id="main">
    % $m->call_next;
    </div>

    <& SELF:footer &>
    </body>

    <%init>
    my $self = $m->base_comp;
    ...
    </%init>

    <%attr>
    body_style => 'standard'
    </%attr>

    <%method title>
    McGuffey Inc.
    </%method>

    <%method header>
    <h2><& SELF:title &></h2>
    </%method>

    <%method footer>
    </%method>

Notice how we provide defaults for each method and attribute, even if blank.

F</products/autohandler> overrides some attributes and methods for the
F</products> section of the site.

    <%attr>
    body_style => 'plain'
    </%attr>
    <%method title>
    McGuffey Inc.: Products
    </%method>

    % $m->call_next;

Note that this component, though it only defines attributes and
methods, must call C<< $m->call_next >> if it wants the rest of the
chain to run.

F</products/index.html> might override a few attributes, but mainly provides
a primary section for the body.

=head1 COMMON TRAPS

=over

=item Do not call $r->content or "new CGI"

Mason calls C<< $r->content >> itself to read request input, emptying
the input buffer and leaving a trap for the unwary: subsequent calls
to C<< $r->content >> hang the server. This is a mod_perl "feature" that
may be fixed in an upcoming release.

For the same reason you should not create a CGI object like

  my $query = new CGI;

when handling a POST; the CGI module will try to reread request input
and hang. Instead, create an empty object:

  my $query = new CGI ("");

such an object can still be used for all of CGI's useful HTML output
functions. Or, if you really want to use CGI's input functions,
initialize the object from %ARGS:

  my $query = new CGI (\%ARGS);

=back

=head1 MASON AND SOURCE FILTERS

Modules which work as source filters, such as C<Switch.pm>, will only
work when you are using object files.  This is because of how source
filters are implemented, and cannot be changed by the Mason authors.

=cut
